1. Technical Field
The present invention relates to a high-strength cold rolled steel sheet having excellent formability, and a plated steel sheet. More particularly, it relates to a high-strength cold rolled steel sheet that has “excellent formability” in such a sense that it has well-balanced tensile strength and elongation (total elongation) as well as well-balanced tensile strength and stretch-flangeability, and a plated steel sheet manufactured by plating the steel sheet. More specifically, the high-strength cold rolled steel sheet or plated steel sheet of the present invention satisfies that the product of tensile strength [TS (MPa)] and elongation [El (%)] is 20,000 or more and the product of tensile strength [TS (MPa)] and stretch-flangeability [λ(%)] is 40,000 or more.
The steel sheet described above can be utilized in wide fields of industry including automobile, electric apparatuses and machinery. Description that follows will deal with a case of using the steel sheet of the present invention in the manufacture of automobile bodies, as a typical application.
2. Background Art
There are increasing demands for high-strength steel sheets for the purpose of improving the fuel efficiency through mass reduction of the steel sheets used in automobiles and improving the safety in the event of collision. Recently, calls for the reduction of exhaust gas emission based on concerns about the global environment add to the demands.
However, high-strength steel sheets are still required to have excellent formability, so as to be formed in various shapes in accordance to the application. In an application where the steel sheet is pressed into a complicated shape, in particular, there is a strong demand for a high-strength steel sheet that combines satisfactory elongation property and stretch-flangeability.
As a high-strength steel sheet having excellent ductility, a TRIP (transformation induced plasticity) steel sheet has attracted special interest recently. The TRIP steel sheet includes residual austenite structure and, when processed to deform at a temperature higher than the martensitic transformation start point (Ms point), undergoes considerable elongation due to induced transformation of the residual austenite (γR) into martensite by the action of stress. Known examples include TRIP type composite-structure steel (TPF steel) that consists of polygonal ferrite as the matrix phase and residual austenite; TRIP type tempered martensite steel (TAM steel) that consists of tempered martensite as the matrix phase and residual austenite; and TRIP type bainitic ferrite steel (TBF steel) that consists of bainitic ferrite as the matrix phase and residual austenite.
Among these, the TBF steel has long been known (described, for example, in Non-Patent Document 1), and has such advantages as the capability to readily provides high strength due to the hard bainitic ferrite structure, and the capability to show outstanding elongation because fine residual austenite grains can be easily formed in the boundary of lath-shaped bainitic ferrite in the bainitic ferrite structure. The TBF steel also has such an advantage related to manufacturing, that it can be easily manufactured by a single heat treatment process (continuous annealing process or plating process).
In a conventional TBF steel, however, satisfactory characteristics have never been attained in stretch-flangeability. The present inventors have recently disclosed, as high-strength/ultrahigh-strength steel sheets that combines high strength and excellent stretch-flangeability, an Al—Mn-based BF steel sheet manufactured by substituting Si with Al and an Al—Mn—Nb—Mo-based TBF steel sheet (Non-Patent Document 2) manufactured by simultaneously adding Nb and Mo to the steel sheet. However, further improvements in characteristics are required in a TBF steel sheet manufactured by the addition of Si of the prior art.    [Non-Patent Document 1] NISSHIN STEEL TECHNICAL REPORT No. 43, December, 1980, p. 1-10)    [Non-Patent Document 2] Akihiko NAGASAKA and five others, “Formability for Forming of Nb—Mo-added TRIP type Bainitic Ferrite Steel Sheet”, CAMP-ISIJ, 2004, Vo. 17, p. 500